The present invention relates to a display apparatus, a display system, and a display control method for the display system.
An information processing system (or apparatus) normally uses a display apparatus as a means for realizing a visual expression function of information. As is well known, a CRT display apparatus is popularly used as such display apparatus.
Since a CRT display apparatus itself has no display memory function, display data must be kept supplied to the display apparatus at all times, and when the supply of display data is stopped, the display operation on the display screen is immediately interrupted.
For this reason, the display control for the CRT display apparatus must constantly execute an image write operation and a display image read-out operation to and from a video memory (to be referred to as a VRAM hereinafter) arranged in the information processing apparatus.
In the case of the above-mentioned CRT display control, since the operation for writing display data in the VRAM to update display information and the operation for reading out data from the VRAM to attain a display operation are independently performed, a program on the information processing system side can write desired display data at an arbitrary timing regardless of the display timing.
However, in general, since a CRT display apparatus has a larger depth in proportion to its display area, the volume of the entire CRT display apparatus increases. That is, the CRT display apparatus suffers a limited installation space, poor portability, and the like, and cannot attain a size reduction.
In view of these problems, a liquid crystal display (to be referred to as an "LCD" hereinafter) is known. The LCD can have an extremely low profile with respect to its display area as compared to that of the CRT display apparatus. Of such LCD, a display using a ferroelectric liquid crystal cell (to be referred to as an FLCD hereinafter) is known.
As one feature of the FLCD, its liquid crystal cell has preservation characteristics of the display state with respect to the applied electric field. That is, the FLCD has a very low-profile liquid crystal cell and an FLC element in the cell can maintain an orientation state after the applied electric field is removed. As a result, such FLC element has bistability, and the FLCD that utilizes the bistability of the FLC element has characteristics of storing the displayed contents. The FLC and FLCD are described in detail in, e.g., Japanese Patent Application No. 62-76357 (U.S. Pat. No. 4,964,699).
Upon driving the FLCD, unlike in a CRT or other liquid crystal displays, since the FLCD can store and continuously display an image, successive refresh periods have a time margin. As a result, in addition to the successive refresh driving operations, a so-called partial rewrite driving operation for updating the display state of only a changed portion on the display screen can be realized.
Meanwhile, the FLCD keeps displaying previous information unless display data is updated. For example, when a host computer or the like hangs up, the previous display state is maintained. Therefore, the conventional display control for a display apparatus cannot solve such problems.
On the other hand, the FLCD performs binary halftone processing for artificially increasing the number of display colors. As typical binary halftone processing, an ED (error diffusion) method that can attain high quality of both natural and character images is known. The ED processing requires an image to have continuity since an error generated in a given pixel diffuses (is distributed) in turn to neighboring pixels.
When the ED method is used, an error from an original image is inevitably generated. Therefore, a change in image quality upon generation of errors caused by an increase in the number of display colors may pose a problem.
Furthermore, some display apparatuses with an intelligence function like in the FLCD have two or more operation modes unlike in a CRT display apparatus which has only one specific display mode and copes with a change in state by simply controlling display data. However, conventionally, confirming the display state on the display state is only one means for confirming the operation mode state, and it is difficult to easily confirm whether or not the power switch of the apparatus is turned off, or whether or not an operation error of the apparatus has occurred.